You will need to add the HP/AP by levels and a trigger to count the levels. Example if at level 1 you get 10hp and 10ap then you will need to tribute(10 stone) or some unused resource to the player once they reach that level so when it respawns it will know the correct amount of hp/ap to give. So the respawn trigger would know if the unit dies and has accumulated 10 stone then respawn unit as lvl 1 and add 10hp/10ap.

You will also need a trigger to activate the re-spawn trigger for each level and deactivate the previous level, such as accumulate 10 stone activate re-spawn trigger lvl 1, accumalate 20 stone activate re-spawn trigger 2 and deactivate re-spawn trigger 1.